Charged particle beam apparatus and dimension measuring method

ABSTRACT

There is provided a charged particle beam apparatus which allows implementation of a high-reliability and high-accuracy dimension measurement even if height differences exist on the surface of a sample. The charged particle beam apparatus includes the following configuration components: An acquisition unit for acquiring a plurality of SEM images whose focus widths are varied in correspondence with the focal depths, a determination unit for determining, from the plurality of SEM images acquired, a SEM image for which the image sharpness degree of the partial domain including a dimension-measuring domain becomes the maximum value, and a measurement unit for measuring the dimension of the predetermined domain from the SEM image whose image sharpness degree is the maximum value.

BACKGROUND OF THE INVENTION

The present invention relates to a charged particle beam apparatus usinga charged particle beam such as an electron beam or ion beam. Moreparticularly, it relates to a charged particle beam apparatus and adimension measuring method which are preferable for measuring thedimension of a specified domain with a high accuracy even if heightdifferences exist within the field-of-view.

In charged particle beam apparatuses representative of which is ascanning electron microscope, a narrowly converged charged particle beamis scanned on a sample, thereby acquiring desired information (e.g.,sample image) from the sample. In the charged particle beam apparatuseslike this, implementation of the high resolution has been progressingyear by year. Of these charged particle beam apparatuses, in electronbeam apparatuses in particular, diffraction phenomenon of the electronsis conspicuous and dominant. As a result, a decrease in the focal depthin accompaniment with the high-resolution implementation is unavoidablein principle. Meanwhile, under the circumstances like this, it is nowrequired to perform a high-reliability dimension measurement with ahigher accuracy. However, in a situation in particular where a pluralityof measurement positions accompany height differences with respect tothe electron beam, it becomes difficult to perform the high-reliabilitydimension measurement.

Conventionally, in order to automatically perform the dimensionmeasurement, the following method has been generally used: Namely, acondition for allowing the best focus to be achieved is found out fromthe entire contrast within the field-of-view including adimension-measuring domain. Then, after setting the focus, the dimensionof a predetermined domain is measured from a SEM image newly acquiredbased on this focusing condition. Also, as a technique for acquiring adeep focal-depth SEM image of a sample which accompanies asperities orheight differences, a technique of acquiring a plurality of SEM imageswith different focuses has been disclosed in JP-A-2002-75263. InJP-A-2002-75263, the method has been disclosed which allows the deepfocal-depth SEM image to be acquired by extracting best-focused imagedomains from these SEM images respectively and superimposing these imagedomains into the one piece of SEM image. Also, in JP-A-11-264726, thefollowing method has been disclosed: Namely, the plurality of SEM imageswith the different focuses are acquired, and the dimension measurementsare performed in length-measuring domains in the respective SEM images.Then, a measurement value whose variation in the measurement resultsacquired for the focus variations becomes the smallest is assumed as thedimension's true value.

SUMMARY OF THE INVENTION

Each of the above-described conventional techniques has the followingproblems: There exists a problem that, if an automatic focus adjustmentis performed in a state where height differences exceeding the focaldepth of an electron-optics system exist within the field-of-view, theentire focusing condition is biased toward the focus of adensely-structured domain within the field-of-view. Accordingly, itturns out that, if the dimension-measuring domain exists outside thedensely-structured domain, the length measurement is performed under acondition which deviates from the appropriate focusing condition. Thissituation results in a decrease in the accuracy and reliability of thelength measurement value. Also, influences by magnetic hysteresis oflenses make unavoidable the inconsistency between the optimum focusingcondition found out by focus search and the actual optimum focusingcondition. This inconsistency causes a certain extent of error to occurin the automatic focus adjustment. Consequently, in accompaniment withthe decrease in the focal depth, this focusing error becomes one causefor the reliability decrease in the length measurement value.

In the technique disclosed in JP-A-2002-75263, the highestimage-sharpness-degree domains detected from the respective SEM imagesare combined thereby to form the one piece of SEM image. At thisprocessing step, however, there exists a possibility that the imagesuperimposing may fail because of influences by noise. Accordingly, insome cases, it is difficult to apply this technique to thehigh-reliability dimension measurement. Also, in the technique disclosedin JP-A-11-264726, no consideration has been given to the focal depth ofthe electron-optics system. This situation requires that, in order toperform the secure dimension measurements, the large number of SEMimages be acquired by varying the focus over steps which are minute morethan necessary. However, when measuring a beam-damage sensitivesubstance such as organic material, it is desirable to complete themeasurement with the smallest possible beam irradiation amount. This isbecause the material which is easily subject to the electron-beam damagewill shrink when exposed to the large amount of beam irradiation, andbecause measuring the accurate dimension becomes impossible.Consequently, in the method disclosed in JP-A-11-264726, the problem ofthe beam damage becomes conspicuous depending on configuration materialsof the sample.

Also, the variations in the dimension measurement value depending on thefocus variations vary depending on a threshold value at the time of edgedetection. The reason for this will be explained referring to FIG. 2.FIG. 2 illustrates line profiles (the entire pattern, left-edge portion,and right-edge portion) of a pattern (i.e., SEM image) acquired underthe appropriate focusing condition and the defocusing condition. In theexample in FIG. 2, if the pixel intensity 100 is defined as thethreshold value, the pattern width seldom varies depending on the focusdifferences. If, however, the pixel intensity 60 is defined as thethreshold value, the pattern width significantly varies depending on thefocus differences. In this way, the focus dependence of the dimensionmeasurement value differs depending on the threshold value of the lineprofiles. Consequently, even if the dimension's true value is predictedfrom the dimension measurement results of the plurality of SEM imageswith the different focuses, the SEM images at that time are notnecessarily based on the appropriate focusing condition. On account ofthis, a practical method has been desired which allows thehigh-reliability dimension measurement value to be acquired with thesmallest possible beam irradiation amount required.

It is an object of the present invention to provide a charged particlebeam apparatus and a dimension measuring method which allow thehigh-reliability dimension measurement value to be acquired under theoptimum focusing condition in a dimension-measuring domain even if theheight differences exist within one and the same field-of-view, or evenif the focal depth is very shallow and the focusing accuracy isinsufficient.

In order to accomplish the above-described object, in the presentinvention, a plurality of SEM images are acquired while varying thefocus with a variation width which is substantially equal to value ofthe focal depth of the electron-optics system. Next, the image sharpnessdegrees of partial domains including a dimension-measuring domain of theplurality of SEM images acquired are evaluated. Moreover, a SEM imagefor which the image sharpness degree of the partial domain is thehighest is selected, then performing the dimension measurement usingthis SEM image. Incidentally, if a plurality of measurement locationsexist within one and the same field-of-view, corresponding partialdomains are set on each measurement-location basis, then evaluating theimage sharpness degrees of the images. Also, there are provided inadvance an input unit for setting a range in which the beam is to beconverged, and a unit for calculating the value of the focal depth ofthe electron-optics system. This makes it possible to acquiremulti-focus SEM images in an appropriate image number.

This makes it possible to grab the images with the minimum beamirradiation amount with respect to a beam-damage sensitive sample aswell. Furthermore, without resetting the focusing, the evaluation of theimage sharpness degrees and the dimension measurement are performedusing the plurality of SEM images themselves which are acquired underthe different focusing conditions. This makes it possible to avoid thefocusing error which occurs when the focusing is newly reset, therebyallowing an enhancement in the accuracy and reliability of the dimensionmeasurement.

According to the present invention, using a high-resolution SEM with ashallow focal-depth, it becomes possible to measure the dimension of aspecified domain with a high reliability and accuracy even if someextent of height differences exist within the field-of-view.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a brief configuration diagram of a scanning electronmicroscope which is an embodiment of the present invention;

FIGS. 2A, 2B, 2C are the explanatory diagrams for explaining the SEMimages resulting from acquiring the same pattern by using the beams withthe different focusing conditions;

FIG. 3 is a diagram for illustrating an example of the processing flowaccording to the present invention;

FIG. 4 is a schematic diagram of the defocus characteristic and thefocal depth of the SEM image;

FIG. 5 is an explanatory diagram in the case of using independentimage-sharpness-degree evaluating methods for each of a plurality ofdimension-measuring domains within one and the same field-of-view;

FIGS. 6A, 6B are diagrams for illustrating evaluation examples of theimage sharpness degrees for the plurality of SEM images;

FIG. 7 is an explanatory diagram for illustrating the focus variationrange for measuring a sample equipped with different heights (i.e.,upper portion and bottom portion of the hole); and

FIGS. 8A, 8B, 8C, 8D are explanatory diagrams for illustratingmeasurement examples of the bottom diameter and top diameter of apattern.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, referring to the drawings, the explanation will be givenbelow concerning embodiments of the present invention.

FIG. 1 is a brief configuration diagram of a scanning electronmicroscope which is an embodiment of the present invention. A voltage isapplied between a cathode 1 and a first anode 2 by a high-voltagecontrol power supply 20 controlled by a computer 40. This voltageextracts a primary electron beam 4 from the cathode 1 as a predeterminedemission current. An acceleration voltage is applied between the cathode1 and a second anode 3 by the high-voltage control power supply 20controlled by the computer 40. This acceleration voltage accelerates theprimary electron beam 4 emitted from the cathode 1, thereby causing thebeam 4 to travel toward a lens system at the subsequent stage. Theprimary electron beam 4 is converged by a condenser lens 5 controlled bya lens control power supply 21. Then, an aperture plate 8 eliminates anunnecessary region of the primary electron beam 4. After that, theprimary electron beam 4 is converged onto a sample 10 as a microscopicspot by a condenser lens 6 controlled by a lens control power supply 22and an objective lens 7 controlled by an objective-lens control powersupply 23. The objective lens 7 is capable of assuming various types ofmodes such as the in-lens system, out-lens system, and snorkel-lenssystem (i.e., semi-in-lens system). Also, the retarding system isassumable which decelerates the primary electron beam by applying anegative voltage to the sample. Moreover, each of the lenses may beconfigured with an electrostatic lens including a plurality ofelectrodes.

The primary electron beam 4 is scanned on the sample 10 in atwo-dimensional manner by a scanning coil 9 controlled by ascanning-coil control power supply 24. A secondary signal 12 such assecondary electrons, which are generated from the sample 10 byirradiation of the primary electron beam 4, travels to a region over theobjective lens 7. After that, the secondary signal 12 is separated fromthe primary electrons by an orthogonal-electromagnetic-field generationdevice 11 for separating the secondary signal, then being detected by asecondary-signal detector 13. The secondary signal 12 detected by thesecondary-signal detector 13 is amplified by a signal amplifier 14.After that, the secondary signal amplified is transferred to an imagememory 25, then being displayed on an image display device 26 as asample image.

A sample stage 15 is capable of displacing the sample 10 in at least twodirections (i.e., X direction and Y direction) within the surfaceperpendicular to the primary electron beam 4. An input device 42 permitsspecification of image grabbing conditions (e.g., scanning speed andacceleration voltage), image output, and image storage into a storagedevice 41. Also, the input device 42 permits specification of alength-measuring domain as well.

This scanning electron microscope also includes a focal-point shiftamount determination unit 51, a SEM-image continuous acquisition unit52, and an image processing unit 53. The image processing unit 53includes an image-sharpness-degree evaluation unit 54 and alength-measurement unit 55.

Based on image forming conditions of the scanning electron microscope,the focal-point shift amount determination unit 51 determines the focaldepth by calculation or the like, thereby calculating focal-point shiftamounts among a plurality of SEM images to be acquired. The SEM-imagecontinuous acquisition unit 52 acquires and stores the series of SEMimages whose focuses are shifted by the focal-point shift amountsdetermined by the focal-point shift amount determination unit 51. Withrespect to the series of SEM images acquired, the image-sharpness-degreeevaluation unit 54 of the image processing unit 53 evaluates the imagesharpness degrees of partial domains including a length-measuring domainspecified by the input device 42, thereby determining a SEM image whoseimage sharpness degree is the highest. With respect to the SEM imagewhose image sharpness degree has been judged to be the highest, thelength-measurement unit 55 performs the dimension measurement of thespecified pattern by using the already-known method as were illustratedin FIGS. 2A, 2B, 2C.

Incidentally, the focal-point shift amount determination unit 51, theSEM-image continuous acquisition unit 52, and the image processing unit53 may be provided outside the computer 40, or may be implemented bypieces of software which operate on the computer 40.

Next, referring to FIG. 3, the explanation will be given belowconcerning details of the processing in the present embodiment.

(1) S11

In this processing, the field-of-view including a dimension-measuringdomain and an initial value of the focusing condition are set.Positioning of the dimension-measuring domain is performed by thematching with the coordinates or template registered in advance. Also,the initial value of the focusing condition is determined as follows:Focus search is performed once with the image sharpness degree of theentire image. Then, a value acquired by defocusing its search result bya predetermined value determined in advance is defined as the initialvalue. The initial value of the focusing condition can also bedetermined as follows: The focus measurement is performed on differentstage-coordinate basis, or on different field-of-view basis, therebycreating in advance a focus map which makes the stage coordinates or thefield-of-views related with the focuses. Then, based on the focus map,the focus value (i.e., initial value) is determined on eachstage-coordinate basis, or on each field-of-view basis.

(2) S12

In this processing, the variation width of the objective-lens currentcorresponding to the focal depth of the electron-optics system isdetermined, then acquiring multi-focus SEM images in a predeterminednumber. The value of the focal depth of a SEM image varies depending onvarious types of factors such as pixel size of the grabbed image, theacceleration voltage, and the resolution. Accordingly, the value of thefocal depth can be defined from the defocus characteristic resultingfrom taking these factors into consideration (i.e., relationshipindicating blurring amount of the image in response to the focusvariation).

FIG. 4 illustrates the defocus characteristic schematically. Thelongitudinal axis in FIG. 4 indicates the resolution relative value of aSEM image at the time when the optimum focusing condition is selected asthe criterion. From this drawing, the range in which the resolutionrelative value is degraded by, e.g., 10% with reference to the optimumfocusing condition can be defined as the value of the focal depth. Ifthe measurement condition is fixed, it is also possible to calculate thevalue of the focal depth of the SEM image in advance, and to registerthe corresponding focal-point variation width (i.e., the variation widthof the objective-lens current) in advance.

When observation magnification is low, the focal depth fd of a piece ofscanning image with the focusing condition fixed is represented by thefollowing expression [1]:fd=A1×(dpix/M)×R×√{square root over (Vacc)}  [1]

Here, A1 denotes a constant, dpix denotes the pixel size, M denotes theobservation magnification, R denotes beam resolution (resolutiondetermined by the beam diameter), and Vacc denotes the accelerationvoltage.

If the observation magnification becomes higher, image resolution of thescanning image turns out to be limited by the beam resolution R. As aresult, the focal depth fd at this time is represented by the followingexpression [2]:fd=A2×R ² ×√{square root over (Vacc)}/(1+0.73×(Ip/B0)×10¹⁴)  [2]

Here, A2 denotes a constant, Ip denotes a probe current, and B0 denotesluminance of the electron gun converted into per-V basis. In the case ofan electric-field emission electron source where the luminance B0 isexceedingly high, the term (Ip/B0) within the expression [2] becomesexceedingly small. As a result, the focal depth fd in thehigh-magnification domain can be represented as the following expression[3] from the practical standpoint:fd=A2×R ² ×√{square root over (Vacc)}  [3]

Incidentally, in the expression [1] to the expression [3], the beamresolution R can be represented by the following expression [4]:Accordingly, the beam resolution R in the expressions [1] to [3] can berepresented in a manner of being replaced by the second term or thirdterm of the expression [4]. Incidentally, A denotes electron wavelength,and α denotes convergence angle (half angle) of the primary beam.R=0.61λ/α=0.75/(α×√{square root over (Vacc)})  [4]

When acquiring a plurality of SEM images with different focuses, thefocal-point shift amounts among the plurality of images are made equalto or somewhat smaller than the values represented by the expressions[1] to [3]. This makes it possible to obtain the maximum focal-depthenlargement effect in the minimum image number.

Based on the calculations in the expressions [1] to [3], the focal-pointshift amount determination unit 51 calculates optimum focal-point shiftamounts from the image forming conditions such as the accelerationvoltage, electron-source luminance, probe current, pixel number,magnification, and beam resolution. The focal-point shift amountdetermination unit 51 is also capable of describing these calculationresults onto a table in advance, and determining the focal-point shiftamount corresponding to an image forming condition by making referenceto the table.

Based on the set values of the focal-point shift amounts, the SEM-imagecontinuous acquisition unit 52 varies the focus on one-image grabbingbasis. Moreover, the SEM-image continuous acquisition unit 52continuously performs the focus control and the image grabbing, thenstoring a series of SEM images with different focuses. The focus controlat this time is capable of assuming the following various types ofcontrol modes: Namely, the focus is controlled with the present focusingcondition selected as the center, the focus is controlled with thepresent focusing condition selected as the end point, or the focus iscontrolled within a focusing range set in advance.

(3) S13

In this processing, with respect to the series of respective SEM imageswith the different focusing conditions acquired in the processing atS12, the image sharpness degrees of partial domains including thelength-measuring domain specified are evaluated, thereby determining aSEM image whose image sharpness degree is the highest. This processingis performed by the image-sharpness-degree evaluation unit 54. In thepresent embodiment, the image sharpness degrees are evaluated by themaximum contrast gradient of the specified length-measuring domain. Thecontrast gradient indicates variation ratio of brightness betweenadjacent pixels with respect to brightness distribution of an image.Namely, as an image becomes shaper, the image exhibits larger contrastgradient (i.e., larger variation ratio of brightness). This is becausethe shaper image accompanies steeper brightness variation at the edgeportion.

The image sharpness degrees, however, can also be evaluated by varioustypes of methods other than the maximum contrast gradient. For example,there exists a method of evaluating the image sharpness degrees byapplying a spatial filter referred to as “differential filter” to thepartial domains to be evaluated, and evaluating the image sharpnessdegrees based on statistical amounts of the pixel values of the partialdomains. In this case, although, as the differential filter, filterssuch as the Sobel filter as primary differential filter and theLaplacian filter as secondary differential filter have been known, thesespatial filters or their modified techniques are also usable. As thestatistical amounts, values are used such as total values, averagevalues, variance values, and standard deviation values of the pixelvalues of the entire partial domains. Then, an image for which thecorresponding value becomes the maximum is assumed as the image havingthe maximum image sharpness degree.

Also, in the present invention, it is also possible to specifyimage-sharpness-degree evaluating methods which are suitable for thestructures of dimension-measuring domains on each domain basis withinone and the same field-of-view. The use of the input device 42 permitsexecution of the specifications of the dimension-measuring domains, theimage-sharpness-degree evaluating methods, and the evaluatingparameters. Next, referring to FIG. 5, the explanation will be givenbelow concerning the case of using independent image-sharpness-degreeevaluating methods for each of a plurality of dimension-measuringdomains within one and the same field-of-view.

FIG. 5 illustrates the case of measuring line width 62 of a pattern anda pattern spacing 64 within one and the same field-of-view. In thiscase, two image-sharpness-degree evaluating partial domains 61 and 63are set. Within the partial domain 61, edges of the image pattern existonly in the X direction. Accordingly, it is possible to calculate theevaluation value by using the differential filter in the X direction. Incontrast thereto, within the partial domain 63, edges of the imagepatterns exist in all the directions. Consequently, it is impossible toperform the correct image-sharpness-degree evaluation only with theevaluation value calculated by the differential filter in the Xdirection or the one in the Y direction. In this case, for example, amethod is employed which defines, as the image-sharpness-degreeevaluation value, the addition value or intensity value (i.e., root ofsquare sum) of the evaluation value by the differential filter in the Xdirection and the one by the differential filter in the Y direction. Inthis way, in the case of setting a plurality of dimension-measuringdomains within one and the same field-of-view, there are some caseswhere, depending on the situation of an image pattern within each of thedomains, it is advisable to set mage-sharpness-degree evaluating methodsindependently.

Furthermore, it is also possible to add a function of judging whether ornot the maximum value of the image sharpness degrees evaluated withrespect to the series of the plurality of SEM images has turned out tobecome a relative maximum value. This function allows implementation ofthe detection if the optimum focusing condition for thedimension-measuring domain were not to be included in the SEM images.For example, if, as illustrated in FIG. 6A, the image sharpness degreesassume an extremum with respect to a series of SEM images with differentfocuses, this situation indicates that a SEM image whose image sharpnessdegree is the maximum has been under the appropriate focusing conditionwithout fail. However, if, as illustrated in FIG. 6B, the imagesharpness degrees assume no extremum with respect to the series of SEMimages with the different focuses, this situation indicates apossibility that the focusing condition is inappropriate. In such acase, the processing can be returned to the processing at S12 bydisplaying the message or changing the initial value. This makes itpossible to enhance the reliability of the measurement result.

Incidentally, in the above-described explanation, the explanation hasbeen given concerning the embodiment of selecting the image whose imagesharpness degree has been found to be the maximum value. The presentinvention, however, is not limited thereto. For example, it is alsopreferable to store a predetermined threshold value in advance, and toselect an image having the image sharpness degree which has exceededthis threshold value. Also, if an image is to be selected under acertain limited condition, it is also preferable to select an imagewhich allows the maximum image sharpness degree to be acquired underthis condition.

(4) S14

In this processing, with respect to the SEM image determined in theprocessing at S13, the dimension measurement of the predetermined domainis performed by using the already-known method. The dimensionmeasurement is performed by the length-measurement unit 55.

(5) S15

If a plurality of measurement locations exist within one and the samefield-of-view, the dimension measurement is performed by repeating theprocessing at S13 and the processing at S14. This method allows thelength measurement to be executed under a focusing condition appropriatefor each length-measuring domain even if height differences exist amongthe plurality of measurement locations. Also, this method reuses theplurality of SEM images which have been grabbed once. Thischaracteristic makes unnecessary the execution of newly beam irradiationfor the measurement, thereby making it possible to perform thehigh-reliability dimension measurement with the minimum beam irradiationamount.

Next, referring to FIG. 7, the explanation will be given belowconcerning an embodiment of the dimension measurement for a sample wherethe measurement locations are different in height. FIG. 7 illustrates anexample of measuring hole upper-portion and hole bottom-portion of asemiconductor having a hole which is deeper than the focal depth of theSEM. Usually, depth (H) of the hole has been already known fromstructure of the device. Accordingly, in this case, N pieces of SEMimages for which the focus is varied by the amount corresponding tovalue of the focal depth are acquired in each of a domain A and a domainB between which the focus positions relatively differ by H. Moreover,when measuring dimension of the upper surface of the hole, themeasurement is performed by selecting an image having the highestsharpness degree from the SEM images of the domain A. Also, whenmeasuring dimension of the hole bottom, the measurement is performed byselecting an image having the highest sharpness degree from the SEMimages of the domain B. This allows the domain A and the domain B to beautomatically determined with the once detected focusing points used ascriterions. This, further, makes it possible to enlarge the focusdomains in the image number N which meets accuracy expectation values ofthe focus points, thereby allowing the high-reliability dimensionmeasurement to be executed.

FIGS. 8A, 8B, 8C, 8D are explanatory diagrams for illustrating anexample of measuring bottom diameter and top diameter of a pattern whichis higher than the focal depth of the electron beam. In this case, thecondition for allowing the focus to be achieved differs depending on adomain to be measured. Consequently, an image for which the specifiedmeasurement domain becomes the sharpest is selected from a plurality ofSEM images with different focuses. This makes it possible toautomatically measure the correct dimension.

Namely, when measuring the bottom diameter of the pattern, asillustrated in FIG. 8A, from a plurality of SEM images acquired byvarying the focus position, an image which is best-focused to the bottomdomain, i.e., the image for which contrast gradient of thedimension-measuring domain at the bottom becomes the maximum value, isextracted, then measuring the bottom diameter d1. As illustrated in across-sectional schematic diagram in FIG. 8B, this image is an image ina state where focusing plane of the electron beam coincides with thebottom domain of the pattern. Accordingly, ridge line of the pattern topis blurred, although the image of the bottom domain is sharp. Also, whenmeasuring the top diameter of the pattern, as illustrated in FIG. 8C,from the plurality of SEM images acquired by varying the focus position,an image which is best-focused to the top domain, i.e., the image forwhich contrast gradient of the dimension-measuring domain on the topbecomes the maximum value, is extracted, then measuring the top diameterd2. As illustrated in a cross-sectional schematic diagram in FIG. 8D,this image is an image in a state where the focusing plane of theelectron beam coincides with the top domain of the pattern. Accordingly,the image of the bottom domain is blurred, although the image of thepattern top is sharp.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A charged particle beam apparatus, comprising: a charged particlesource, a charged particle optical system for converging a primarycharged particle beam emitted from said charged particle source, andscanning said primary charged particle beam on a sample, andsecondary-signal detection means for detecting a secondary signalgenerated from said sample by said scanning of said primary chargedparticle beam, a sample image being acquired using said secondary signaldetected by said secondary-signal detection means, wherein there areprovided acquisition means for acquiring a plurality of sample imageswith respect to field-of-view including a dimension-measuring domain,said plurality of sample images having different focusing conditions,determination means for evaluating image sharpness degrees of partialdomains including said dimension-measuring domain with respect to saidplurality of sample images acquired, and determining, from saidplurality of sample images, a sample image whose image sharpness degreeis the highest, and measurement means for measuring dimension of saidpredetermined domain from said sample image determined.
 2. The chargedparticle beam apparatus according to claim 1, wherein focus variationwidths of said plurality of sample images are determined incorrespondence with values of focal depths of said plurality of sampleimages.
 3. The charged particle beam apparatus according to claim 1,wherein said image sharpness degrees are evaluated using maximumcontrast gradient of said dimension-measuring domain.
 4. The chargedparticle beam apparatus according to claim 1, further comprising meansfor specifying image-sharpness-degree evaluating methods or evaluatingparameters, said image-sharpness-degree evaluating methods or evaluatingparameters being independent of each other for each of a plurality ofdimension-measuring domains within said field-of-view.
 5. A dimensionmeasuring method for measuring dimension from a sample image formedusing a secondary signal, said secondary signal being generated from asample by scanning of a primary charged particle beam, said dimensionmeasuring method comprising the steps of: setting a dimension-measuringdomain within field-of-view of said sample image, determining anin-focus position for the entire field-of-view of said sample image, anddefining a position as an initial focus position, said position beingacquired by a defocusing from said in-focus position by a predeterminedamount, acquiring a plurality of sample images while varying thefocusing from said initial focus position on a predetermined-amountbasis, said plurality of sample images being focused before and behindsaid in-focus position, evaluating image sharpness degrees of partialdomains including said dimension-measuring domain with respect to saidplurality of sample images, and determining, from said plurality ofsample images, a sample image whose image sharpness degree is thehighest, and performing said dimension measurement using said sampleimage determined.
 6. The dimension measuring method according to claim5, wherein focus variation widths of said plurality of sample images aredetermined in correspondence with values of focal depths of saidplurality of sample images.
 7. The dimension measuring method accordingto claim 5, wherein said image sharpness degrees are evaluated usingmaximum contrast gradient of said dimension-measuring domain.
 8. Thedimension measuring method according to claim 5, whereinimage-sharpness-degree evaluating methods or evaluating parameters arespecified, said image-sharpness-degree evaluating methods or evaluatingparameters being independent of each other for each of a plurality ofdimension-measuring domains within said field-of-view.
 9. A scanningelectron microscope, comprising: a charged particle source, a scanningdeflector for scanning a charged particle beam emitted from said chargedparticle source, and an objective lens for converging said chargedparticle beam, wherein sharpness degrees of a plurality of images areevaluated, and dimension of a pattern formed on a sample is measuredusing an image whose sharpness degree is higher than a predeterminedvalue, said plurality of images being acquired when focusing conditionof said charged particle beam is varied.